Optical time domain multiplexing is widely employed in high bit rate transmitter modules and is required for the use of modern components in systems with bit rates of 40 Gbps or higher. With increasing use of high bit rate components, optical systems are depending more and more on optical time domain multiplexing and its associated RZ (return to zero) data format. Improving the qualities of the data stream of high bit rate transmissions in RZ format is desirable and ever increasingly important.
Traditionally, non-carrier suppressed optical signals do not compensate for neighboring pulse interaction, which arises when return to zero format data is simply encoded by a straight amplitude modulation or attenuation of the optical carrier without taking into account the phase of the carrier. Carrier suppression is achieved by ensuring that the phase of the carrier in neighboring RZ pulses differs by π (or differs by an odd integer of π). For all intents and purposes with respect to achieving carrier suppression, a phase difference of π and an odd integer of π are equivalent, Carrier suppressed RZ data format (often referred to as CS-RZ format) in optical fiber has potential advantages over traditional conventional transmission formats. Such benefits are larger power margins, higher possible input power, and excellent transmission performance under relaxed system conditions; see A. Sano, Y. Miyamoto, “Performance Evaluation of Prechirped RZ and CS-RZ Formats in High-Speed Transmission Systems With Dispersion Management” Journal of Lightwave Technology, vol. 19, No. 12, December 2001, and Yanjun Zhu, W S Lee, C Scahill et al, “16-channel 40Gb/s Carrier-suppressed RZ ETDM/DWDM Transmission over 720 NDSF Without Polarization Channel Interleaving,” OFC'2001, ThF4-1, 2001.
Referring to FIG. 1, conventional carrier suppressed modulation is described. An input optical signal being an input carrier pulse array in RZ format propagates along waveguide portion 5. A graph illustrating the array of pulses and their associated intensity, carrier and phase is shown for both the input optical signal and the output optical signal. It should be noted that the illustration represents the carrier at a much lower frequency relatively speaking in comparison with the RZ pulse width than that actually used. High rate optical RZ data format typically is 40 Gb per second or higher, however, typical optical carrier frequencies can be in the range of hundreds to thousands of times that. The input optical signal traversing waveguide portion 5 is made up of RZ pulses which are out of phase by π with neighboring pulses. The input optical signal enters an optical attenuator or optical modulator 10 via an input 7. The modulator 10 encodes data into the pulse array by attenuating appropriate pulses of the RZ stream. The output optical signal emerging from an output 8 of the modulator 10 is a carrier suppressed RZ (CS-RZ) data stream. This CS-RZ data stream propagates along a waveguide portion 9 as an output optical signal. The illustration associated with the output optical signal shows the data modulated onto the carrier by variation in the intensity of the RZ pulses, while the phase relationship of the neighboring pulses remains fixed at a difference of π between successive pulses, resulting in a CS-RZ stream.
Referring to FIG. 2, the optical signals typically produced by a conventional optical time domain multiplexing (OTDM) module is described. The optical data stream from a conventional OTDM module is not carrier suppressed. The RZ data formal commonly used in OTDM implementations normally has no built in requirement for the phases of neighboring pulses to have any relationship. In fact, due to the nature of typical OTDM pulse multiplexing, the phase relationship between successive pulses is unknown and constantly changing due to changing environmental condition for example variations in temperature.
The conventional OTDM module has a input waveguide portion 15 over which an input optical signal made up of RZ pulses in a pulse arrays propagates. FIG. 2 shows an illustration of a typical RZ pulse array for the input signal. Each RZ pulse is separated in time by the absence of any signal for a duration at least equal to the duration of each pulse. The carrier of the input is also depicted in the illustration. For the purposes of discussion the pulse period of the input pulse array is Δt. The optical pulses are split by an optical splitter 17 which is set to split the power of the optical signals over two tributaries, an upper tributary, tributary A 50 and a lower tributary, tributary B 60, so that the optical signals may be separately modulated. Signals set to traverse tributary A 50, propagate over a waveguide portion 18 to an input 21 of an optical attenuator or optical modulator 20. The optical modulator 20 encodes data into the pulse stream by attenuating pulses. A modulated optical signal emerges from an output 22 of the optical modulator 20, and traverses waveguide portion 25 to enter through an input 31 of an optical delay element 30. The optical delay element 30 is a longer path or optical waveguide portion through which the optical signals must propagate in comparison with that of the lower tributary, namely tributary B 60. The modulated and delayed optical signal emerges from an output 32 of the optical delay 30 to traverse an optical waveguide portion 35. Signals set to traverse tributary B 60, propagate over a waveguide portion 19 to an input 71 of an optical attenuator or optical modulator 70. The optical modulator 70 encodes data into the pulse stream by attenuating pulses. A modulated optical signal emerges from an output 72 of the optical modulator 70, and traverses waveguide portion 75. The optical signals are such that once they are combined at an optical combiner 40, the attenuated pulses from each tributary will interleave one another in time (time domain multiplexing). Such a condition is met when, if the period of the input pulse array is Δt, the delay caused by the optical delay element to the optical signal traversing tributary A 50 causes a time delay difference of Δt/2 or an odd multiple thereof. FIG. 2 depicts this offset by way of an illustration of the pulses of each tributary just before the optical signals are combined. The optical signals traversing tributary A 50 and tributary B 60 are combined by the optical combiner 40 and emerge along a waveguide portion 45 connected to the output of the combiner 40 as a resulting time domain multiplexed signal. The intensity carrier signal and phase of this output is illustrated in FIG. 2. It should be noted that the phase difference between successive RZ pulses is x.
The resulting optical signal is not CS-RZ format. Suppose the phase of pulses in the input pulse array is that of a continuous sinusoid. The phase difference between the adjacent pulses at the output of the combiner is determined by the fine optical path difference between the two tributaries. Only if the optical path difference coincidentally is exactly is some odd multiple of λ/2 would the phase difference of adjacent RZ pulses be equal to π. As described above, the carrier period in time is much smaller than the bit period of the modulated signal, in fact the carrier period is on the order of about 5·10−15 seconds. In conventional OTDM modules, it is very difficult to produce an optical path difference between two tributaries which is precisely some odd number of λ/2. The optical path difference also varies with time due to for example periodic variations in temperature or temperature gradients across the module. The carrier phase difference between adjacent pulses is random and for practical purposes is unknown and unpredictable.
Given the known benefits of carrier suppressed data formats, it would be desirable for there to be a method and an apparatus which provides optical time domain multiplexing which produces optical data signals in a carrier suppressed RZ data format.